　　一般时通常用来描述事情的永久状态或常常发生的事。例如：

　　◆ SIMPLE PRESENT Sandy PLAYS well with her friends.

　　◆ SIMPLE PAST Sandy PLAYED well with her friends yesterday.

　　◆ SIMPLE FUTURE Sandy WILL PLAY well with her friends tomorrow.

　　进行时表示动作正在进行：

　　◆ PRESENT PROGRESSIVE Sandy IS PLAYING soccer.

　　◆ PAST PROGRESSIVE Sandy WAS PLAYING soccer yesterday.

　　◆ FUTURE PROGRESSIVE Sandy WILL BE PLAYING soccer tomorrow.

　　上面的例子表示说Sandy 正在PLAYING soccer ，而一般现在时：Sandy plays soccer,则是强调Sandy 比较经常或者她知道如何plays soccer 。在一般的定义中我们通常避免使用进行时而使用一般时：

　　◆ Wrong: Cherenkov radiation is light that particles ARE EMITTING when they ARE TRAVELING faster than the effective speed of light in any medium.

　　◆ Right: Cherenkov radiation is light that particles EMIT when they TRAVEL faster than the effective speed of light in any medium.

　　表达一般状态的动词一般不用进行时，例如know or signify：

　　◆ Wrong: This inscription IS SIGNIFYING the emperor’s birth.

　　◆ Right: This inscription SIGNIFIES the emperor’s birth.

该知识点的练习册

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